This invention concerns an improved apparatus and method for high-voltage conditioning, or "spot-knocking," electron tubes including television cathode ray picture tubes having certain resistive arc-suppression means.
Electron guns for television cathode ray picture tubes usually comprise a series of discrete, electrically conductive discs or tubular element contiguous to each other and aligned on a common axis. In multi-gun assemblies, each gun may comprise a series of electrically discrete electrodes, or, the electrodes of the gun which have functions in common may be physically combined, or "unitized." The electrodes receive voltages of a predetermined potential to establish electrostatic fields therebetween for forming and shaping the electron beam, and, in the main focus lens section of such guns, for imaging a beam cross-over to provide small, symmetrical beam "spots" on the screen. The electrodes of the main focus lens of such guns are typically closely spaced and have widely varying potentials thereon; e.g., potentials ranging from 4 to 30 kilovolts or more.
Consequent to the process of manufacture of the metallic electrodes, in which shearing, stamping, and coining are typical process, surface imperfections such as burrs, spikes and other detritus may project from the surface of the electrode. Because of the close spacing of the electrodes and the wide disparity in potential on adjacent electrodes, such projections can become point sources for inter-electrode arcing. Arcing can cause visible flashes and popping noises discomforting to the viewer, and possible destruction of the cathode ray tube itself and/or its ancillary components and circuits.
It is a well-known manufacturing practice to induce beneficial arcing between electrodes to expunge such projections by a process commonly known as spot-knocking. Typically, the electrode nearest the screen, commonly known as the anode electrode, is charged with a high potential, while the adjacent electrode is held at ground potential. Arcing is deliberately induced between the two electrodes by techniques such as raising the anode potential to higher than normal voltage, typically in the range of 40 to 50 kilovolts; pulsing the voltage; and/or applying a radio frequency component. The arcing that results removes the projections and in consequence, no arcing ordinarily occurs when the gun is operated at its normal potential.
FIG. 1 is a simplified schematic diagram of a spot-knocking circuit 10 in common use. A cathode ray tube 11 is symbolically represented as having two electrodes 12 and 13 enclosed in the evacuated envelope and between which spot-knocking is to be induced. Electrode 12 represents the "lower end" electrodes nearest the base of the tube 11; that is, the heater and cathodes, the prefocusing electrodes, and one or more of the main focusing electrodes, all of which are typically electrically interconnected and connected to ground through a lead-in pin 14. Electrode 13 represents the electrode nearest the faceplate and is commonly termed the anode electrode, receiving as it does the anode voltage on the faceplate screen as supplied by a high voltage source 15 through an anode button 16, all as is well-known in the art. A positive voltage from a spot-knocking high-voltage source 18, typically in the range of 35 to 50 kilovolts, is conducted to anode button 16 through a current-limiting resistor 20. When high-voltage source 18 is activated, the difference in potential between electrodes 12 and 13 results in the desired beneficial interelectrode arcing, or spot-knocking. The type of power source shown can be described as "unipolar"; that is, a single potential of one polarity is provided, which in the example shown, is of positive polarity.
A problem that restricts the utility of the spot-knocking circuits shown by FIG. 1 is attributable to the inherent capacitance 22, indicated by the dashed lines, of the picture tube, its electrodes and associated conductors. This inherent capacitance puts a limit on a magnitude of the voltage that can be supplied for spot-knocking purposes. If the voltage is raised much higher than 50 kilovolts, for example, the energy stored in the inherent capacitance 22 increases to a high level. As a result, when spot-knocking arcing does occur, the release of stored energy is of such magnitude that, rather than being beneficial, the resultant arcing can be highly destructive both to the gun and the cathode ray tube itself.
A problem arises in attempting to spot-knock cathode ray tubes having certain resistive arc-suppression means. A tube having such means is disclosed in U.S. Pat. No. 4,101,803 to Retsky et al., assigned to the assignee of this invention. It is manifest that the electron gun of a cathode ray tube having means to suppress arcing (as the Retsky et al. U.S. Pat. No. 4,101,803 invention effectively demonstrates) would not ordinarily be compliant to the prior art spot-knocking process.
Nakanishi et al.--U.S. Pat. No. 3,736,038 discloses a method for spot-knocking electron tubes such as a cathode ray picture tube. The disclosure is addressed to rendering a standard spot-knocking process effective with respect to more than two of the electrodes of an electron gun. That is, spot-knocking is alleged to be accomplished not only between a first electrode at the anode potential and a second electrode adjacent thereto, but also between the second electrode and a third electrode adjacent thereto. This additional function is the result of the insertion of resistive means in the circuit that causes a rise of potential of the second electrode concurrent with the spot-knocking arc effective to induce a spot-knocking arc between the second and third electrodes. The potential for arcing between lead-in pins would seriously restrict the magnitude and hence efficacy of the spot-knocking potential that could be employed.
In U.S. Pat. No. 4,124,263, Neuber et al. discloses a process for the high-voltage conditioning of cathode ray tubes. The process is described as being applicable to the high-voltage conditioning of the type of cathode ray tube having an electron gun with more two main focus electrodes (termed a "tri-potential focus type") that require relatively high operating potentials supplied through the tube base pins. One aspect of the invention comprises a special tube socket adapted for receiving the tube base, and a container of high-dielectric-strength fluid into which the assembled socket and base is immersed to inhibit inter-pin arcing during high-voltage conditioning. Major disadvantages in the process include the high cost of the dielectric fluid, and the contaminative nature of the fluid which mandates that the high-voltage conditioning process be confined to an area apart from the main production line. Also, the requirement to utilize containers of the fluid renders the process cumbersome.
"Flashover in Picture Tubes and Methods of Protection" is the title of a paper by A. Ciuciura that appeared in the Journal of the Institution of Radio and Electronic Engineers in March 1969. Flashover protection is described as being provided in picture tubes by spark gaps assisted by series resistors.
U.S. Patent No. 1,532,228 (Great Britain) discloses spark gap and discharge path arrangement for a television picture tube or other cathode ray tube. Apparatus is provided comprising a cathode ray tube having an envelope of dielectric material with inner and outer conductive coatings, an ultor electrode coupled to the inner coating, a high voltage focus electrode in close proximity to the ultor electrode, and a plurality of low voltage electrodes in proximity to the focus electrode. The first spark gap has a first terminal coupled to the focus electrode and a second terminal directly coupled to the outer coating, which coating is at a reference potential. The spark gap has a breakdown potential lower than the breakdown potential between the focus electrode and an adjacent low voltage electrode. A plurality of second spark gaps have respective first terminals coupled to respective low-voltage electrodes, and respective second terminals directly coupled to the outer coating along a path separate from the direct coupling of the second terminal of the first spark gap to the outer coating. The general object of the invention is stated to be to prevent discharge currents and associated high voltage from one spark device from undesirably coupling back through the common return to affect the operation of other spark gaps.